In digital power metering systems it is often desirable to determine the spectral content of the power signal of interest, i.e., the spectral components that are superimposed on the fundamental frequency of the power supply system, which is typically at 50 Hz or 60 Hz. In discrete time systems this is usually done by sampling the analog power signal, converting the samples to digital values using an analog-to-digital (“A/D”) converter and then performing a transformation from the time domain to the frequency domain. The tool often used to perform this transformation is the Fast Fourier Transform (FFT), although other methods may be employed.
In order for the transformation to occur error-free, it is necessary that an integer number of cycles of the signal be sampled to provide data for the FFT or any other technique used for the transformation. If this condition is not met, a phenomenon known as spectral leakage occurs. Spectral leakage produces false spectral components, i.e., a signal consisting of a single frequency should produce a single spectral line, but if leakage occurs false components will appear in the spectrum.
The spectral information is used when compensating a system to reduce harmonic content and for other troubleshooting purposes. The typical digital power meter utilizes an analog-to-digital (A/D) converter and a microprocessor, and thus all analysis is done in the discrete time or digital domain. The signal is digitized by the A/D converter operating at a sampling rate which is determined by an adjustable frequency digital clock. In order to sample an integer number of cycles of the signal (assuming the sample rate is held constant during the sampling window), it is necessary that the sample rate and the frequency of the signal be integrally related. The required sample rate is determined by measuring the frequency of the input signal and then multiplying this by some integer such that the Nyquist requirement and other system constraints are met. Because the adjustable sample clock does not have infinite precision, it is not possible to set it to the required frequency for some input frequencies.